At present, optical discs having a recording capacity of approximately 50 GB can be commercialized for consumer use by a Blu-ray Disc™ standard using a blue-violet semiconductor laser. Hereafter, in optical disc, an increase in capacity has been also required to the same degree as an HDD (Hard Disk Drive) capacity of 100 GB to 1 TB.
However, in order to realize such an ultrahigh density in an optical disc, there is the need for high-density techniques based on different new methods which is distinguished from high-density techniques based on a reduction in the wavelength of a laser and an increase in NA of an objective lens.
During research on the next-generation storage techniques, a hologram recording technique for recording digital information using holography has attracted attention.
The hologram recording technique refers to a technique in which signal light having information of page data two-dimensionally modulated by a spatial light modulator is superimposed on reference light in the inside of a recording medium, and refractive index modulation is brought about within the recording medium by an interference fringe pattern which is generated at this time, to thereby record information on the recording medium.
During information reproduction, the irradiation of the recording medium with reference light used during recording causes holograms recorded in the recording medium to act like diffraction gratings to thereby generate a diffracted light. This diffracted light is reproduced as the same light inclusive of the recorded signal light and phase information.
The reproduced signal light is detected two-dimensionally at high speed using a photo-detector (camera) such as a CMOS or a CCD. In this manner, the hologram recording technique can record two-dimensional information on an optical recording medium at a time using one hologram, and can reproduce the information. Since a plurality of pieces of page data can be overwritten at a certain place of the recording medium, the technique can be used for recording and reproducing high-capacity and high-speed information.
As a hologram recording and reproducing technique, for example, JP-A-2006-172582 (PTL 1) is disclosed. This publication discloses that “in addition, a mechanical shutter 25 that opens and closes a beam during recording is disposed between a half-wave plate 24 and a polarization beam splitter 26, and a recording time is determined depending on the opening and closing timing of the mechanical shutter 25.”
In addition, as the hologram recording and reproducing technique, for example, JP-A-2007-40414 (PTL 2) is disclosed. This publication discloses that “a pattern providing such polarized light as that in which all the signal light is shielded by a polarizing plate 14 for a period between data recording and data recording is displayed on a spatial modulator (SLM) 13 that performs spatial modulation on a signal light 100. Thereby, since the hologram recording material 50 is not irradiated with the signal light 100 between the recording and the recording, it is possible to prevent useless recording on the hologram recording material 50 from continuing. In addition, in this case, the spatial modulator 13 functions as a shutter, but its operation is rapid and a vibration does not occur therein. Therefore, it is possible to record a precise signal, and to make a transmission rate higher.”